The invention pertains to a method for the manufacture of ethylene in a pyrolysis furnace, and more particularly to a method for modifying an ethylene furnace to accept and process a feedstock of crude oil and crude oil fractions containing pitch.
The production of olefins, in particular ethylene, is achieved conventionally by the thermal cracking of petroleum hydrocarbon feedstocks using natural ga liquids, (NGL""s) such as ethane or by using the naphtha or gas oil fractions produced from a crude distillation column operating above atmospheric pressure. More recently, the trend in some regions is toward designing crackers to accommodate the use of heavier feedstocks, such as vacuum gas oils. These heavier feedstocks, however, foul tubes in convection section preheaters and downstream equipment by coke deposition. Typical process temperatures at the exit of the convection section first stage preheaters range from about 200-400xc2x0 C., thereby completely vaporizing the feedstock within the convection section, or in heavy feed cases such as gas oil and vacuum gas oil, finally and completely vaporizing the feedstock externally as it proceeds toward the second stage preheaters through a mix nozzle with superheated steam as described in U.S. Pat. No. 4,498,629 to Grondman, entitled xe2x80x9cApparatus for Vaporization of a Heavy Hydrocarbon Feedstock With Steam.xe2x80x9d
U.S. Pat. No. 5,580,443 discloses a process for cracking low quality feedstock such as a heavy natural gas-liquid, which is an associated oil occurring in small quantities with the production of gas from gas fields. The process is described as processing the feedstock through a first stage preheater within the convection zone to a vapor/liquid separator external to the convection zone after being mixed with superheated steam, a second stage preheater within a convection zone, and finally to the radiant zone. The feedstock is cracked by separating and removing in a vapor-liquid separator a portion of heavy fractions from the first stage preheater section, and subsequently returning the vaporized portion of the feedstock to the second stage preheater before subjecting the feedstock to pyrolysis. The temperature and pressure within the first stage preheater tubes are maintained within a range such that those fractions of the feed which would otherwise cause coking problems in the tubes are kept in liquid state, while fractions unlikely to cause coking problems are fully evaporated. Typical exit temperatures from the first preheater section range from 150xc2x0 C.-350xc2x0 C. in order to avoid vaporizing the coke generating fractions within the tubes.
The gas-liquid mixture exiting the first preheater section is described in U.S. Pat. No. 5,580,443 as within a ratio of 60/40 to 98/2. This ratio can be adjusted by the addition of superheated dilution steam at a point between the exit port of the first preheater section and prior to entry in a vapor-liquid separator. Once in the vapor-liquid separator, the heavy unevaporated liquid fractions are removed and discharged from the system, while the gaseous fraction is passed through a gas delivery line, mixed with superheated dilution steam again, and then passed to the second preheater. In the second preheater, the gas is heated up to a temperature just below the temperature at which cracking is promoted, after which it passes into the radiant section and is cracked.
It would be desirable to process feeds other than heavy natural gas-liquids through a pyrolysis furnace for the manufacture of ethylene. Desirable feeds include crude oil or the long residue from the bottoms of a crude oil atmospheric column. Crude oil feed is derived from oil fields wherein 60% or more of the production extract in liquid form is a crude oil. A heavy natural gas-liquid stream is in a gaseous or supercritical state in the ground, which condenses into a liquid as it reaches surface temperatures and pressures. Processing a crude oil feedstock or the long residue of a crude oil atmospheric column through a pyrolysis furnace under the temperature conditions described in U.S. Pat. No. 5,580,443, and in particular at a temperature ranging from 150xc2x0-350xc2x0 C. in a first preheating stage, or at any temperature at which those fractions likely to cause coking problems remain in liquid state and those fractions unlikely to coke the tubes are fully evaporated, would be disadvantageous because at the lower temperatures at which heavy natural gas-liquids are processed, 150xc2x0 C.-350xc2x0 C., insufficient fractions of vaporized crude oil or long residues are recovered, resulting in reduced yields of desirable olefin production from these feedstocks.
The heavy ends of crude oil and long residue cannot be vaporized under typical olefins pyrolysis furnace convection section conditions. The heavy ends of crude oil and long residue are normally removed by distillation, and the lighter vaporizable fractions from a distillation, most commonly the naphtha or gas oil fractions, are used as the feed for olefins pyrolysis plants. This distillation preparation step for crude oils and long residue requires additional capital and adds additional operating cost to the process.
There is now provided a process for pyrolyzing a crude oil feedstock or crude oil fractions containing pitch feedstock, and a pyrolysis furnace, comprising feeding the crude oil or crude oil fractions containing pitch feedstock to a first stage preheater within a convection zone, wherein said crude oil or crude oil fractions containing pitch feedstock is heated within the first stage preheater to an exit temperature of at least 375xc2x0 C. to produce a heated gas-liquid mixture, withdrawing from first stage preheater the gas-liquid mixture to a vapor-liquid separator, separating and removing the gas from the liquid in the vapor-liquid separator, and feeding the removed gas to a second preheater provided in the convection zone, further heating the temperature of said gas to a temperature above the temperature of the gas exiting the vapor-liquid separator, introducing the preheated gas into a radiant zone within the pyrolysis furnace, and pyrolyzing the gas to olefins and associated by-products.
There is also provided a process for pyrolyzing a crude oil or crude oil fraction containing pitch feedstock comprising feeding said feedstock to a first stage preheater within a convection zone of an olefins pyrolysis furnace, heating said feedstock to a temperature at which non-coking fractions and a portion of coking fractions are fully evaporated to a gas, and the remaining portion of coking fractions are maintained in a liquid phase, while maintaining a gas/liquid weight ratio of said crude oil in the first stage preheater within a range of 60/40-98/2, withdrawing the gas/liquid mixture from the first stage preheater and feeding the gas/liquid mixture to a vapor/liquid separator, separating the gas from said gas/liquid mixture in the vapor-liquid separator and removing said gas from the vapor-liquid separator, feeding said removed gas to a second stage preheater in the convection zone, heating said gas in the second stage preheater, and feeding a second stage preheated gas to a radiant heat transfer zone in the pyrolysis furnace.
There is also provided a process for pyrolyzing a crude oil or crude oil fractions containing pitch feedstock comprising feeding a crude oil or crude oil fractions containing pitch feedstock to a first stage preheater in a convection zone of an olefins pyrolysis furnace, heating the crude oil or crude oil fractions containing pitch feedstock in the first stage preheater to a temperature of at least 400xc2x0 C. while simultaneously feeding a dilution gas in the first stage preheater, wherein the gas-liquid weight ratio ranges from 60/40-98/2.
Each of the above processes are also used to process a long residue and any crude oil fractions containing pitch.
The process of the invention allows one to feed a crude oil or crude oil fractions containing pitch feedstock into the convection zone of a pyrolysis furnace without having to decoke the tubes in the convection zone any sooner than the radiant tubes of a furnace. The process of the invention extends the capability of an ethylene furnace to flash a feedstock (a feed of crude oil or crude oil fraction containing pitch) at a higher temperature (e.g. 480xc2x0 C.) that is not generally achievable at the bottoms of a vacuum distillation column under normal operating conditions (about 415xc2x0 C.), thereby allowing one to recover a higher fraction of the crude oil or crude oil fractions containing pitch as vapor useful for cracking in the radiant heat transfer zone in a pyrolysis furnace than that recovered through atmospheric or vacuum distillation columns. The process of the invention also has the advantage of processing a crude oil or crude oil fractions containing pitch feed without having to first subject the crude oil or crude oil fractions containing pitch feed to fractionation, thereby allowing one to process a cheaper source of feedstock in a pyrolysis furnace. Finally, the large quantity of higher boiling fractions in crude oil or crude oil fractions containing pitch, unlike heavy natural gas liquids, wet the inner surfaces of the tubes in the convection zone at suitable linear velocities under the operating temperatures described herein, thereby making crude oil or crude oil fractions containing pitch a suitable feed and minimizing the formation of coke within the convection zone tubes.